Pressure dependence of the conduction-electron-spin-resonance linewidth of theαandβphases of di-bis(ethylene- diothiolo)tetrathiafulvalene triiodide

We report conduction-electron-spin-resonance linewidth (ΔH) measurements of the α and β phases of the organic conductor di-bis(ethylenediothiolo)tetrathiafulvalene triiodide [(BEDT-TTF${)}_2$ ${\mathrm{I}}_3$] in the 80–300 K temperature range under applied pressures of up to 5 kbar. ΔH increases under pressure in contrast to the predictions of the Elliot formula for the spin relaxation in metals. The pressure derivative d(lnΔH)/dP is -5.5±1%/kbar and 9.8±1%/kbar for the α and β phases of (BEDT-TTF${)}_2$ ${\mathrm{I}}_3$ respectively.